Coiled plate type heat exhangers are well known in the art, and the assignee of the present invention is the assignee of several patents which disclose invention designs of coiled plate heat exchangers utilized in condensors designed to recover the waste heat from refrigeration systems to heat water for diary farms and the like. Examples of these are U.S. Pat. Nos. 4,146,089, 4,179,902 and 4,305,456. As disclosed in these prior patents, a coiled plate heat exchanger may be constructed by seam welding a pair of plates along their edges, resistance welding a plurality of spot welds across the face of the plate, and inflating the plate under pressure to expand the sides away from each other and create a "pillowing" effect to dramatically increase the size of the passageway between the plates. The plate may be coiled into an annular configuration before it is inflated, and once inflated fixed in that annular configuration to reduce the size of the heat exchanger. This plate heat exchanger design has been quite successful, and is well received in the industry.
As shown in these prior patents, the coiled plate may then be placed in a container, refrigerant pumped through the plate, and water permitted to convection flow through the container as it is heated by the refrigerant, the refrigerant being cooled by the water and condensed for reuse by the refrigeration system.
In some other applications, the assignee has also manufactured and sold a variation of this coiled plate type heat exchanger which utilizes a center or middle plate, with two outer plates surrounding the middle plate to thereby create a passageway between each of the outer plates and the middle plate. This dual flow condensor is described in U.S. Pat. No. 4,484,623 also owned by the assignee of the present invention. As disclosed therein, this sandwich plate type heat exchanger may be coiled and also used in a condensor unit to heat water as shown in the patents listed above, with the exception that refrigerant may flow through one side of the plate and water may flow through the other side of the plate, there thereby being no need to immerse the entire plate in a container which is water tight.
While both types of construction are more than adequate in providing separation between the refrigerant and the water, safety standards have been established by Underwriters Laboratories Inc. for heat exchangers to be used in referigerant heat recovery systems for heating potable water which require designs which minimize the risk of contamination of the water. One such design which meets the criteria established by UL is a design which provides double wall separation between the potable water and the refrigerant, with a vented interface between the two interior double walls so that if leakage develops in the heat exchanger wall which contains the refrigerant, it will not pressurize the interface or exert pressure on the other interior wall which could, over time, result in failure of the other wall thus allowing refrigerant to leak into the potable water. UL's standards further require that the interface be maintained at near atmospheric pressure, and that the venting path be continuous over the entire length of the heat exchanger. The continuity of the venting path is determined by pressurizing the interface with water at a pressure not higher than 10 psig, and then testing for leaking from the interface within a one minute time limit. The passageway which conducts the refrigerant must be capable of withstanding a hydrostatic pressure equivalent to five times the maximum rated design pressure as the water side is maintained at atmospheric pressure. The passageway for the water must be capable of withstanding a hydrostatic pressure of at least 300 psig, which is the same requirement as UL has established for water heater storage tanks.
These standards are quite difficult to meet as the typical refrigerant system is rated at 400 psig, which requires a test at more than 2,000 psig. In an expanded plate which has been "pillowed", the plate to be pillowed must be sufficiently thin that pillowing can be easily achieved, and yet sufficiently strong to hold its weld at the spot welds. Furthermore, as different alloys of stainless steel are typically utilized in the different plates of an assembly, and their thicknesses are different as explained above, and the spot welder which applies the welds to the plates generates a great deal of heat in performing its welding process which is difficult to dissipate and yet maintain a reasonable speed of manufacture, the stainless steel plates have a tendency to gather or wrinkle as two or more plates are welded together. As can be appreciated, what may be a minor irritation when welding two plates together is somewhat worsened when three plates are attempted in the sandwich plate construction, much less the severe situation created by attempting the spot welding process with four plates.
Despite these many obstacles, the inventors herein have succeeded in developing a method of manufacturing a dual-walled coiled plate type heat exchanger with a vented interface which meets the requirements of UL and has been certified for UL listing. Briefly, the method consists of taking four rectangular sheets of stainless steel, punching the necessary holes in the sheets to accommodate fluid tight connections as taught by U.S. Pat. No. 4,484,623, and taking the four plates two at a time and resistance seam welding a pair of seams the length of each pair along the central portion of each pair which in effect joins each pair individually together. During this welding step, it has been found that placing a roller or other support underneath the plates as they are being welded will deflect the plates and serve to prevent wrinkling or gathering of the plate as the welder heats the plate during the welding process. This is very effective in minimizing wrinkling and permits the welder to move continuously in a reasonable manufacturing speed across the plates, without inordinate stopping of the process to permit cooling or any other means used to cool the plates. Next, the two pairs of plates are aligned and welded with spot welds in a uniform pattern across the surface of the plates. These spot welds extend through all four plates, and serve to join all four plates together.
After the four plates are joined across their surface by the plurality of spot welds, the seams are welded along the outer periphery of the plate, and baffles are also seam welded into the assembly on both sides of the middle seam welds. At this point, the assembly is sheared between the middle seams which thereby creates a pair of heat exchanger plate assemblies, each assembly having all four plates welded along three of its sides, with the fourth side adjacent the seam weld being vented between the middle plates. Each of the assemblies may then be separately processed by rolling the assembly into an annular configuration, assembling the fluid connections through the holes previously punched, placing the assembly in a fixture, inflating both passages in both sides of the assembly at the same time and under substantially the same pressure, and then welding the assembly into the annular configuration to hold it after removal from its fixture. It is noted that simultaneous inflation of both passages tends to force the middle plates into close thermal contact. It has been found that there still remains sufficient separation to permit the flow of fluid towards the middle seam, and hence vent the interface, as is required to meet the UL standard. To enhance this separation, the inventors have found that the middle plates may be roughened prior to assembly so that the middle plates have a tendency to remain further separated when the passages are inflated.
In one embodiment of the invention, the four plate assembly may be built from 18 gauge and 20 gauge 316L alloy stainless steel on one side, and 18 gauge and 20 gauge 304 alloy stainless steel on the other. Typically, the heavier gauge plates are on the outside as they are subjected to the intense pressures during testing.
The foregoing brief description is merely a synopsis of the invention, the details of which may be more fully understood by referring to the drawings and description of the preferred embodiment which follow.